An accumulator absorbing a pulsation of a hydraulic pressure generated in a hydraulic piping or the like is constituted of an outer shell branched from and connected to the hydraulic piping via an oil port, and a diaphragm bladder separating an internal space of the outer shell into a pressure introduction chamber in the side of the oil port and a gas chamber in the opposite side. A pulsation of the hydraulic pressure within the piping is introduced into the pressure introduction chamber via a fluid inlet and outlet port of the oil port, whereby the bladder is displaced in a direction in which the hydraulic pressure of the pressure introduction chamber and a pressure of N2 gas charged into the gas chamber from a gas charging hole in the outer shell so as to function as a cushion gas are balanced, absorbs the pulsation, and levels a system hydraulic pressure within the piping.
However, in the case that the bladder is made of a rubber member, the cushion gas charged within the gas chamber permeates through the bladder with time little by little due to gas permeability of the rubber member. Accordingly, during a longtime use, an amount of cushion gas in the gas chamber is significantly reduced, and there is a risk that it is impossible to sufficiently achieve the function.
Therefore, in conventional, in order to cope with the problem mentioned above, there has been developed a technique of improving resistance to gas permeability of the bladder by making the bladder in a structure of interposing a composite layer (a gas barrier laminated body) 100 obtained by laminating a center elastic layer 101 constituted of a polyamide resin or a mixture of a polyamide resin and a polyolefine resin, a gas shielding layer 102 disposed in an outer side thereof and constituted of an ethylene-vinyl alcohol copolymer, and an outer elastic layer 103 disposed in a further outer side thereof and constituted of a polyamide resin or a mixture of a polyamide resin and a polyolefine resin within a rubber layer 110, as shown in FIG. 5.
That is, in the conventional art, when a layer thickness of the center elastic layer 101 is set to A, a layer thickness of the gas shielding layer 102 is set to B, and a layer thickness of the outer elastic layer 103 is set to C, A/B is set to be not less than 0.1 and not more than 3.5, C/B is set to be not less than 1.0 (refer, for example, to Japanese Patent No. 2700592 or the like). Further, a rate of elasticity of the center elastic layer 101 and the outer elastic layer 103 is set to be 100 to 1000 MPa, and a rate of elasticity of the gas shielding layer 102 is set to be 1800 to 3300 MPa (refer, for example, to Japanese Patent No. 2700594 or the like). Further, on the basis of the laminated structure mentioned above, it is possible to effectively prevent the cushion gas in the gas chamber 5 from being reduced due to the gas permeability in the bladder 3, and it is possible to make the durability of the bladder 3 excellent.
In the meantime, in the case of the accumulator for a motor vehicle, since an ambient temperature at night in a winter season becomes below freezing point in some areas, it is necessary to consider a temperature environment between −40° C. and −120° C. However, in accordance with the conventional art mentioned above, an excellent gas shielding function can be achieved in a temperature range from a room temperature to about 80° C., but a flexibility of the gas shielding layer 102 constituted of the ethylene-vinyl alcohol copolymer is reduced in a low temperature environment of not more than 0° C., so that a load of the shielding layer 102 becomes large due to an application of repeated bending and stretching deformation, and there is a problem that a crack and a peeling are generated and a gas shielding function is deteriorated.
The present invention is made by taking the problem mentioned above into consideration, and a technical object thereof is to provide an accumulator with a eature which can improve a durability against a repeated bending and stretching motion of a bladder under a low temperature environment of not more than 0° C., and can effectively prevent a cushion gas in a gas chamber from being reduced.